The present disclosure generally relates to speckle reduction concepts for light illumination systems that require lasers for use as the light source for light diffusing optics such as a light diffusing fiber (LDF).
LDFs require a certain level of spatial coherence from their illumination source to achieve high coupling efficiency at a low cost. The spatial coherence requirement implies that laser sources are required for each color (red, green and blue). Typically, the laser sources are more coherent than the level of coherency required by an LDF. An LDF requires some level of spatial coherence, but does not require a spatially single mode beam because it is typically configured for multimode operation. In order to make the laser source inexpensive, this extra spatial tolerance can be utilized as alignment tolerance and the laser source fills several of the modes of the LDF. After a short length of propagation down the LDF fiber, light is scattered from the modes excited by the laser source into all of the higher order modes supported by the LDF. The number of modes supported can exceed 100, depending on the fiber core diameter and the numerical aperture (NA) of the LDF.
The problem with having a moderate amount of both spatial and temporal coherence is that the LDF itself contains a noticeable amount of speckle. In addition, the areas illuminated with the emitted light from the LDF may also contain have speckle. Speckle, resulting from the optical interference of the beam with itself, can be perceived as undesirable noise associated with the operation of the light illumination system. Further, the amount of speckle associated with a given light illumination system can have various levels of severity. In addition, the sensitivity of consumers and other individuals using a particular light illumination system to speckle also can vary. Certain individuals, for example, can find even small levels of speckle associated with a given light illumination system (e.g., those that rely on laser light sources in conjunction with LDFs) undesirable compared to other light production systems.
Consequently, there is a need for light illumination systems that minimize or eliminate speckle effects with cost effective schemes, particularly systems employing laser light sources and LDFs.